1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly, to an image forming apparatus that shifts to a power saving mode after a lapse of a predetermined time from the end of printing.
2. Description of the Related Art
Many conventional electrophotographic copiers and printers adopt a heating roller fixing device of a contact heating type with high efficiency and safety or a film heating system as a heat fixing portion.
The heating roller fixing portion of the heating roller fixing system is mainly composed of a heating roller (fixing roller) serving as a heating rotational body and an elastic roller serving as a heating rotational body that is in pressure contact therewith. The roller pair is rotated so that a recording material (recording sheet, electrostatic recording sheet, electrofax sheet, print sheet, etc.) serving as a workpiece on which an unfixed image (hereinafter, referred to as a toner image) is formed and born is introduced into a fixing nip portion that is a pressure contact nip portion between the roller pair and is conveyed through the fixing nip portion, and thus, the toner image is fixed as a permanent image on the surface of the recording material with heat from the heating roller and the pressure of the fixing nip portion.
The heat fixing portion of the film heating system is a device in which a heat resistive film (fixing film), which is a heating rotational body, is slidingly conveyed while it is brought into contact with a fixed heater, such as a ceramic heater, using a pressing rotational body (elastic pressure roller), and a recording material bearing a toner image is introduced into a fixing nip portion, which is a contact pressure nip portion formed between the heater and the pressing rotational body, with the film interposed therebetween, and is conveyed together with the film to thereby fix the toner image on the recording material as a permanent image with heat applied from the heater via the film and the pressure of the fixing nip portion.
These heat fixing portions perform heating fixation in such a manner that a temperature detection device, such as a thermistor, is disposed at the heating roller or the ceramic heater, serving as a heating member, to control the fixing temperature to a desired degree. In this case, if it is controlled to a constant fixing temperature, the amount of heat applied to the sheet changes depending on the degree of warming of heat fixing portion, in particular, the pressure roller, which sometimes cause fixing defect or hot offset. Therefore, various methods for controlling the fixing temperature depending on the degree of warming of the heat fixing portion have been proposed.
For example, Japanese Patent Laid-Open No. 2002-169407 discloses a method of measuring a print stop time and setting a fixing temperature from the measurement, and Japanese Patent Laid-Open No. 08-69205 discloses a method for setting an optimum fixing temperature from a printing time and number-of-prints information.
Power saving of the apparatus has recently been required, and in particular, reduction of power consumption during standby has been required. Therefore, a power saving mode (hereinafter referred to as a sleep mode) is widely adopted in which power consumption is reduced by, for example, a method of shutting off power supply of part of the electric circuit of the apparatus during standby.
However, the sleep mode also reduces power supply to a CPU or the like that controls the apparatus, which makes it impossible to monitor the temperature of the heat fixing portion and perform timer measurement and calculating operation of a number-of-prints counter or the like, etc. during the sleep mode. Thus, there is no information indicating the temperature changes of the heat fixing portion and the degree of warming of the heat fixing portion, such as the stop time and the number of prints; therefore, a fixing temperature for a print job after returning from the sleep mode is determined only from the detection temperature, at the start of printing, of a temperature detection device disposed at the heating member. This precludes appropriate fixing temperature control based on the degree of warming of the heat fixing portion, thus posing the situation of insufficient heat supply to sheets to cause fixing defect or the situation of excessive heat supply to cause hot offset.
Thus, there is also a method in which a temperature detection device, such as a thermistor, is disposed on the surface of the pressure roller, and the fixing temperature is determined depending on the degree of warming of the pressure roller. However, an additional temperature detection device and a detection circuit may be needed for temperature detection, thus increasing the cost.
Furthermore, increasing power supply to the CPU to allow temperature monitoring and the calculating operation of the number-of-prints counter etc. for controlling the fixing temperature makes it impossible to reduce the power consumption during standby, which makes it impossible to respond to the request for power consumption.